Introduction to Waste heat recovery
● With the growing trend of increases in fuel prices over the past decades as well the rising concern regarding global warming, engineering industries are challenged with the task of reducing greenhouse gas emissions and improving the efficiency of their sites.
● In this regard, the use of waste heat recovery systems in industrial processes has been key as one of the major areas to reduce fuel consumption, lower harmful emissions and improve production efficiency. Sources of waste heat mostly include heat loss transferred through conduction, convection and radiation from industrial products, equipment and processes and heat discharged from combustion processes.
● Waste Heat Recovery is the process of capturing waste heat (thermal) energy for beneficial purposes. Recaptured heat energy is potentially useful for generating electricity.e.g. Space heating, water supply and green houses
● A waste heat recovery unit (WHRU) is an energy recaptured unit with a heat exchanger that transfers heat from process outputs at high temperature to another part of the process for some purpose, which increases performance of plant to improve efficiency. Heat recovery is a complex system, which requires many parameters to be considered. The exit temperature of the waste heat as well and the application of the regenerated waste heat is key factors in heat recovery systems
● Waste heat is heat energy which is generated in a process from combustion fuel or chemical reaction, and then dumped into the environment even though it could still be reused for some useful and economic purpose. The essential quality of heat is not the amount but rather its "value". The strategy of how to recover this heat depends in part on the temperature of the waste heat gases and the economies involved. Large quantity of hot flue gases is generated from Boilers, Kilns, Ovens and Furnaces.
● If some of this waste heat could be recovered, considerable amount of primary fuel could be saved. The energy lost in waste gases cannot be fully recovered but loss can be minimized.
Need of Waste Heat Recovery :
● A large quantity of Flue gases at high temperatures is produced by different parts of the heavy industries, They are purified internally by purifying equipment like Electrostatic Precipitators and are left into atmosphere. At the present condition the heat energy of flue gases is wasted. If we are able to trap the heat and there by using the heat to produce steam, an efficient electric power generation can be achieved.
● The total heat generated in these industries is not utilizing full-fledged but 45% is discharged through the exit gases from the preheater and through chimney and the gases which are releasing in to atmosphere through this waste heat are, NOx & Sox, soot particle and particulates, Carbon dioxide, Mercury, Vaporized heavy metals etc.
● These gases are causes environmental pollution and dangerous to human health creating problems people who are leaving surroundings to these industries and indirectly it is forming 10-12% of global warming effect. If these are able to trap the heat and there by using the heat to produce steam, an efficient electric power generation can be achieved by this waste heat.
● The waste heat recovery in electric power is most effective method, because of the advantages offered by the Waste Heat Recovery power plant are as follows :
1. Clean power generation
2. No production of green house gases (ex: CO2)
3. No air polluting gases (NOx, SOx) etc.
4. Reduces 10-12% of Global warming effect.
● The waste heat recovery plants can be constructed on the same basis of the thermal power plant with the only difference that flue gases for the production of heat in boilers replace the burning of coal. This difference makes the recovery plant most efficient and clean over conventional plant for same power rating.
Opportunity for Waste Heat Recovery:
Based on estimation of heat losses in selected applications, several trends were identified regarding opportunity area and RD&D need for waste heat recovery. The following are the opportunity areas are:
1. Low temperature waste heat source : Based on a 25 C reference most uncover waste heat is at low temperature About 60% of waste heat losses are at a temperature below 230°C.
2. System already including waste heat recovery that can be further optimized to reduce heat loss : The extent of heat recovery system is often constrained by cost and temperature limit for the heat recovery system. In many cases cement preheater kilns and recuperative glass furnace, exhaust gases existing the recovery device are still in the medium to high temperature range. This represent an opportunity for additional waste heat recovery. Opportunity are also available to maximum quality of heat recovered since facilities are often to use dilution air to lower the temperature of waste heat steam.
3. High temperature system where heat recovery is less common : There are the segments where waste heat recovery is less common this is due to barrier such as chemical constituent in the exhaust gases that interface with heat exchange as well as limitation on economics of scale for small waste heat stream.
4. Alternate waste heat sources typically not considered for waste heat recovery : This focus on combustion and process exhaust gases, where as alternate source of waste bent were also found to be significant The alternatives are heat radiated, convicted and conducted from heat products as well as heat loss in side walls, and after pyro-processes where slag or after material are solidified to protect the vessel walls.
High - 650°C and higher
Medium - 230°C to 650°C
Low - 230 C and lower
Limitations of Low Temperature Waste Heat Recovery :
1. Corrosion of the heat exchanger surface: As water vapor contained in the exhaust gas cools, some of it will condense and deposit corrosive solids and liquids on the heat exchange surface. The heat exchanger must be designed to withstand exposure to these corrosive deposits. This generally requires using advanced materials, or frequently replacing components of the heat exchanger, which is often uneconomical.
2. Large heat exchange surfaces required for heat transfer : Heat transfer rates are a function of the thermal conductivity of the heat exchange material, the temperature difference between the two fluid streams, and the surface area of the heat exchanger. Since low temperature waste heat will involve a smaller temperature gradient between two fluid streams, larger surface areas are required for heat transfer. This limits the economics of heat exchangers.
3. Finding a use for low temperature heat : Recovering heat in the low temperature range will only make sense if the plant has a use for low temperature heat. Potential end uses include domestic hot water, space heating, and low temperature process heating. Other options include using a heat pump to "upgrade" heat to a higher temperature to temperatures. serve a load requiring higher
Waste heat recovery in thermal power plant :
● In thermal power plant, the heat from the waste hot stream is recovered either in the form of sensible heat or latent heat. The Heat Losses are consider in term of Quality and Quantity
● Depending upon the various type of process carried in industries, waste heat can be rejected at virtually any temperature from that of chilled cooling water to high temperature waste flue gases from an industrial furnace or kiln. As higher is the temperature, higher the quality of heat so it is more cost effective heat recovery.
● The sensible heat is covered from air or water which is a heat carrying medium the water which is used for cooling purpose in cooling circuit or from flue gases passes from air preheater and economizer to the chimney. Typical examples of use would be preheating of combustion air, or pre-heating boiler feed water or process water. With high temperature heat recovery, a cascade system of waste heat recovery may be practiced to ensure that the maximum amount of heat is recovered at the highest potential.
● An example of this technique of waste heat recovery would be where the high temperature stage was used for air pre-heating and the low temperature stage used for process feed water heating or steam raising.
● Heat recovery from heat treatment furnaceor the exhaust gases are leaving the furnace is quantity or the amount of available waste heat can be calculated using the equation shown below.
Q = V × p × Cp × ∆T
Where
Q = is thetatal heat content in kJ
V = is the volume flow rate of the substance in m3/hr
p = is density of the flue gas in kg/m3
Cp = is specific heat of the working fluid in kJ/kg℃
∆T = is the temperature difference in °C between the final highest temperature in the outlet (Tout) and the initial temperature in the inlet (Tin) of system.
Cp = Specific heat of flue gas in kJ/kg/℃
● Depending on the type and source of waste heat and in order to justify which waste heat recovery system can be used, it is essential to investigate the amount and grade of heat recoverable from the process.
Difference between Waste Heat Sources and Quality:
Source → Quality
1. Heat in flue gases. → The higher the temperature, the greater the potential value for heat recovery.
2. Heats in vapour streams → As above but when condensed, latent heat also recoverable.3. Heat lost from exterior of equipment by Convective and radiation → Low grade - if collected may be used for space heating or air preheat.
4. Heat losses in condenser circuit by cooling water → Low grade - useful gains if heat is exchanged with incoming fresh water.5. Heat losses in providing chilled water or in the disposal of chilled water. → a. High grade if it can be utilized to reduce demand for refrigeration. b. Low grade if refrigeration unit used as a form of heat pump.
6. Heat stored in products leaving the processes → Quality depends upon temperature.7. Heat in gaseous and liquid effluents leaving process. → Poor if heavily contaminated and thus requiring alloy heat exchanger.
